1. Field of the Invention
The present invention relates to a method for producing a ferroelectric thin film that is controlled to have preferential crystal orientation in the (100) plane with a simple process.
Priority is claimed on Japanese Patent Application No. 2011-110680 filed on May 17, 2011 and Japanese Patent Application No. 2012-073402 filed on Mar. 28, 2012, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, ferroelectric thin films utilized as capacitors and piezoelectric elements are widely developed so as to satisfy requirements for downsizing electronic elements.
Lead zirconate titanate (PZT) is a ferroelectric material that has a perovskite structure and exhibits excellent ferroelectric properties. Using a chemical solution deposition (CSD) method utilizing sol-gel solution, it is possible to achieve homogeneous film composition in the plane of a substrate with inexpensive film-forming process. Therefore, the CSD method is known as a method of obtaining film-capacitors utilizing the PZT as a material for ferroelectric thin films.
Where the ferroelectric film is formed by the CSD method utilizing sol-gel solution, platinum or iridium having crystal faces oriented in the (111) direction may be formed as a base electrode on a substrate. By forming the ferroelectric thin film on the base electrode formed on the substrate, it has been possible to obtain ferroelectric thin films having crystals with preferred orientation in the (111) plane depending on the (111) orientation of the base substrate.
Such ferroelectric thin films having a (111)-preferred orientation are appropriately utilized in an IPD (Integrated Passive Device), a non-volatile memory or the like because of their high withstand voltage and life-time reliability.
On the other hand, conventional methods known to form a film having preferred crystal orientation in (100) plane or (110) plane on the base electrode oriented in the (111) direction include using a seed layer composed of a material different from the ferroelectric thin film and introducing a buffer layer composed of a material different from the ferroelectric thin film so as to suppress the structural influence of the bottom electrode. Ferroelectric thin films having a preferred orientation in (100) plane are appropriately used in the application such as actuators because of a large e31 piezoelectric constant. Ferroelectric thin films having a preferred orientation in (110) plane are appropriately used in the application such as capacitors because of the large dielectric constant.
A method for producing a ferroelectric film including introduction of a buffer layer is disclosed, for example, in Japanese Unexamined Patent Application First Publication No. 2011-29399 (Claim 7, paragraphs [0003], [0022] to [0026], [0039] and FIG. 1). This method for producing a ferroelectric film includes: a step of forming a base film oriented to a predetermined crystal plane on a surface of a substrate; a step of forming a carbon film on the base film; a step of forming an amorphous thin film containing ferroelectric material on the carbon film; and a step of forming a ferroelectric film on the base film by heating and thereby crystallizing the amorphous thin film. The ferroelectric film formed by this method is oriented to a crystal plane different from the predetermined crystal plane, and the ferroelectric material is composed of at least one of five types of materials, a first type material composed of perovskite structure and bismuth layered-structure oxide, a second type material composed of superconducting oxide, a third type material composed of tungsten-bronze structure oxide, a fourth type material composed of at least one selected from a group consisting of CaO, BaO, PbO, ZnO, MgO, B2O3, Al2O3, Y2O3, La2O3, Cr2O3, Bi2O3, Ga2O3, ZrO2, TiO2, HfO2, NbO2, MoO3, WO3, and V2O3, a fifth type material containing the fourth type material and SiO2, and a sixth type material containing the fourth type material, SiO2, and GeO2. In Japanese Unexamined Patent Application First Publication No. 2011-29399 (hereafter referred to as Patent Reference 1), the crystal orientation of the ferroelectric film formed on the carbon film is controlled by controlling a thickness of the carbon film formed as a buffer layer. In the practical embodiment described in Patent Reference 1, the orientation of the PZT is controlled to have a (111) plane +(001) plane orientation where a thickness x of the diamond like carbon (DLC) film formed as the carbon film is in the range of 0 nm<x<10 nm, the PZT is controlled to have a (001) plane orientation where the thickness x of the DLC film is 10 nm, the PZT is controlled to have a (001) plane +(110) plane orientation where the thickness x of the diamond like DLC film is in the range of 10 nm<x<100 nm, the PZT is controlled to have a (110) plane orientation where the thickness x of the DLC film is 100 nm, and orientation of the PZT is controlled to have weak (110) orientation where the thickness x of the DLC film is larger than 100 nm
The Patent Reference 1 also describes constituting a buffer layer by stacking LaNiO3 strongly self-oriented in the (001) direction on the base electrode having (111) orientation, and forming a PZT film on the LaNiO3 buffer layer, and thereby achieving a PZT film having (001) orientation.
However, the method described in the above-described Patent Reference 1 requires complicated processes including introduction of the seed layer and the buffer layer. In addition, the presence of such a seed layer and a buffer layer may cause deterioration of properties of the ferroelectric thin film and may cause contamination or the like.
As a method for controlling crystal orientation of ferroelectric thin film, for example, Japanese Unexamined Patent Application, First Publication No. H06-116095 (hereafter referred to as Patent Reference 2) discloses a method including: coating a precursor solution of PZT or PLZT on a platinum substrate having crystal faces oriented in (111) direction, and heating the substrate to form a ferroelectric thin film, wherein the substrate being coated with the precursor solution is firstly subjected to a heat treatment at a temperature of 150 to 550° C. to achieve a designated crystal orientation, and is subsequently subjected to firing at a temperature of 550 to 800° C. to crystallize the precursor, thereby selectively orienting the crystal plane of the thin film to a specific direction depending on the heat treatment temperature (Claims 1 to 4, paragraphs [0005], [0006], FIG. 1).
In Patent Reference 2, crystal orientation of ferroelectric thin film is controlled depending on the temperature range of heat treatment corresponding to preliminary firing, thereby forming a ferroelectric film having controlled crystal orientation on the base electrode directly without interposing a seed layer or a buffer layer. Specifically, Patent Reference 2 describes that the crystal orientation is controlled such that preferred orientation in the (111) plane is achieved by the heat treatment at 150 to 250° C., preferred orientation in the (111) plane and (100) plane is achieved by a heat treatment at 250 to 350° C., and preferred orientation in the (100) plane and (200) plane is achieved by a heat treatment at 450 to 550° C.
However, applicability of the above-described method of Patent Reference 2 is limited or performance of the film is insufficient in a practical application since the achieved preferred orientation includes plural orientations, for example, (111) plane and (100) plane, or (100) plane and (200) plane.
When the Pb-containing perovskite type ferroelectric film has thin thickness, there is a case that crystal nuclei cannot be generated in sufficient density, resulting in anomalous growth of large crystals, and making it difficult to control the grain diameters of crystals. The ferroelectric film with such anomalously grown crystals is sometimes inferior in insulation property, thereby including problems in its quality.
An object of the present invention is to provide a method for producing ferroelectric thin film that enables achievement of a ferroelectric thin film that is controlled to have preferred crystal orientation in the (100) plane with a simple process without forming a seed layer or a buffer layer.
The other object of the present invention is to provide a method for producing a ferroelectric thin film by which the thickness of a ferroelectric film having preferred crystal orientation in the (100) plane can be arbitrary adjusted depending on its intended use.
Still another object of the present invention is to provide a method of producing a ferroelectric thin film that enables control of crystal diameters of a ferroelectric thin film having a preferred crystal orientation in the (100) plane.